Niemann Pick type C (NPC) is an autosomal recessive neurodegenerative lipid storage disorder characterized by a block in the intracellular mobilization of lysosomal cholesterol. The NPC cellular phenotype can be distinguished from normal by analysis of LDL induced Filipin staining and cellular cholesterol esterification. Efforts to localize the gene responsible for NPC have limited the candidate region to a 1 cM interval on chromosome 18q. To further delineate the location of the NPC gene, we have analyzed YACs spanning this interval for their ability to complement the NPC phenotype. Three overlapping YACs containing the neomycin resistance gene were introduced individually into phenotypically NPC-like CHO cells by yeast spheroplast fusion. G-418 resistant clones were analyzed by STS content to determine the extent of the YAC DNA present in each cell line. Preliminary data from both Filipin staining and cholesterol esterification assays indicate correction of the NPC-like phenotype in two clones derived from YAC D5 (telomeric in the interval) which were positive for all STSs tested. In contrast, clones derived from D5 which were incomplete by STS analysis and clones derived from two other YACs (centromeric in the interval) retained the NPC-like phenotype. This is the first suggestion that a genomic clone from chromosome 18 contains the NPC gene. Current work focuses on spheroplast fusion to human and mouse NPC cell lines to confirm the ability of YAC D5 to correct the NPC phenotype.